1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon layer, and more particularly to a mask for a sequential lateral solidification of an amorphous silicon layer and a crystallization method thereof.
2. Discussion of the Related Art
In general, silicon is classified into amorphous silicon (a-Si) and polycrystalline silicon (p-Si) according to the phase of silicon. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in switching devices of liquid crystal displays (LCDs) using the glass substrate. Unfortunately, amorphous silicon thin film transistors (a-Si TFTs) have relatively slow display response times due to their poor electrical properties that limit their suitability for large area LCDs.
In contrast, polycrystalline silicon thin film transistors (p-Si TFTs) provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices, such as laptop computers and wall-mounted television sets. A thin film of polycrystalline silicon is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grains boundaries, the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be advantageous.
One method of crystallizing amorphous silicon into polycrystalline silicon is the sequential lateral solidification (SLS) method. The SLS method uses the fact that silicon grains tend to grow laterally from the interfaces between liquid phase and solid phase silicon such that the resulting grain boundaries are perpendicular to the interfaces. With the SLS method, amorphous silicon is crystallized by irradiating a specific area with a laser beam through a mask such that the melted silicon forms laterally grown silicon grains upon re-crystallization. Next, the mask is moved, and the laser irradiates a new area. This process is then repeated until the desired area is crystallized.
FIG. 1 is a schematic view showing a configuration of an apparatus for a sequential lateral solidification method according to a related art.
In FIG. 1, an apparatus for a sequential lateral solidification (SLS) method includes a laser generator 36, a first optical unit 40, a mask assembly 32, a second optical unit 42 and a stage 46. The laser generator 36 generates and emits a laser beam 34. The intensity of the laser beam 34 can be adjusted by an attenuator (not shown) in the path of the laser beam 34. The laser beam 34 is then condensed by the first optical unit 40 and directed onto the mask assembly 32. The laser beam 34 is processed through the mask assembly 32 and then again condensed by the second optical unit 42. The condensed laser beam 34 is irradiated onto a specific region of a substrate 44 on the stage 46 movable along the x and y directions. Even though not shown in FIG. 1, a buffer layer is formed on the substrate and an amorphous silicon layer is formed on the buffer layer. A dehydrogenation process may be performed for the amorphous silicon layer before the crystallizing process.
The mask assembly 32 includes a mask 38 having a transmissive region “A” through which the laser beam 34 passes and a shielding region “B” which screens the laser beam 34. Accordingly, the irradiated shape of the amorphous silicon layer is dependent on the mask 38. Because the size of the laser beam 34 and the size of the mask 38 are limited in the present technology, it is impossible to crystallize the entire amorphous silicon layer simultaneously. Therefore, to crystallize the amorphous silicon layer by using the apparatus of the SLS method, the following steps are repeated: the laser beam 34 irradiates the substrate through the mask 38; and the substrate 44 or the mask assembly 32 moves by several micrometers or several tens micrometers. Thus, the amorphous silicon layer is crystallized one section at a time, and the crystallization process is repeated for each section.
FIGS. 2A to 2C are schematic plane views showing crystallization states of an amorphous silicon layer by an SLS method according to the related art. For example, an apparatus for the SLS method of FIG. 1 is used.
In FIG. 2A, a laser beam 34 (FIG. 1) irradiates am amorphous silicon layer 52 through a mask 38 (FIG. 1). Accordingly, only a first irradiation region of the amorphous silicon layer 52 is melted. This irradiation region includes a first, second, and third portions D, E, and F corresponding to the transmissive region A (FIG. 1) of the mask 38 (FIG. 1). Here, laser beam energy (FIG. 1) is within the complete melting regime where the amorphous silicon layer 52 is completely melted.
After the laser beam 34 (FIG. 1) first irradiates the substrate a silicon grain 58a grows laterally from an interface 56 between liquefied silicon and adjacent solid silicon to form polycrystalline silicon. The silicon grain 58a grows toward a center of the liquefied silicon along a perpendicular direction to the interface 56. The maximum distance of grain growth depends on several factors such as the energy density of the laser beam, the substrate temperature, and the state of the amorphous silicon layer. In general, the maximum distance of grain growth is within a range of about 1 μm to about 2 μm. If the width of the transmissive region globally change A (FIG. 1) of the mask 38 (FIG. 1) is greater than twice the maximum distance of grain growth, a region of microcrystalline silicon is generated between the grains growing from the interfaces 56. To prevent the generation of a region of microcrystalline silicon, the width of the transmissive region A (FIG. 1) of the mask 38 (FIG. 1) should be less than twice of the maximum distance of grain growth. Polycrystalline silicon corresponding to the transmissive region A (FIG. 1) of the mask 38 (FIG. 1) is formed in the amorphous silicon layer 52 by the first irradiation of the laser beam.
In FIG. 2B, the laser beam 34 (FIG. 1) irradiates the amorphous silicon layer 52 (FIG. 2A) a second time through the mask 38 (FIG. 1). The second irradiation region of the laser beam 34 (FIG. 1) partially overlaps the first irradiation region including the first, second, and third portions D, E, and F (FIG. 2A), which were crystallized by the first irradiation of the laser beam 34 (FIG. 1), to prevent grain growth independent of crystallization by the first irradiation of the laser beam 34 (FIG. 1). When the maximum distance of grain growth is within a range of about 1 μm to about 2 μm, the substrate 44 (FIG. 1) or the mask assembly 32 (FIG. 1) of an apparatus for a SLS method should move by a distance less than about 1 μm. Accordingly, the second irradiation region includes a peripheral portion of the first irradiation region and the amorphous silicon layer adjacent to the first irradiation region. The polycrystalline silicon and the amorphous silicon in the second irradiation region are melted and then crystallized as described above. Therefore, the grain of the first irradiation region continues to grow laterally in the second irradiation region.
As shown in FIG. 2C, the amorphous silicon layer 52 (FIG. 2A) of one section is crystallized by repeating the irradiation and the movement, and thereby the entire amorphous silicon layer 52 (FIG. 2A) is crystallized.
Even though the SLS method produces large size grains, a crystallization direction is limited to either the vertical the horizontal directions. Moreover, because the mask or the substrate moves only by a several micrometers along the crystallization direction, the time required to move the mask or the substrate take a significant portion of the total process time. This significantly decreases manufacturing throughput. Therefore, a mask having a new pattern shape is suggested.
FIG. 3 is a schematic plane view showing a pattern shape of a mask for a sequential lateral solidification method according to the related art.
In FIG. 3, a mask 62 for a sequential lateral solidification (SLS) method includes first to sixth transmissive regions “N1” to “N6” having an equal rectangular shape. The number of the transmissive regions may be more or less than six. The six transmissive regions N1 to N6 are disposed along horizontal and vertical directions H and V. The first to third transmissive regions N1 to N3 are disposed in a stairstep arrangement along the horizontal direction H. Similarly, the fourth to sixth transmissive portions N4 to N6 are disposed in a stairstep arrangement along the horizontal direction H. The first and fourth transmissive regions N1 and N4 are spaced apart from each other along the vertical direction V. Similarly, the second and fifth transmissive regions N2 and N5, and the third and sixth transmissive regions N3 and N4 are spaced apart from each other along the vertical direction V. The first to third transmissive regions N1 to N3 meet each other at their ends. The fourth to sixth transmissive regions N4 to N6 also meet each other at their ends. After first irradiation of the laser beam 34 (FIG. 1), the mask 62 or the substrate 44 (FIG. 1) moves along the horizontal direction H by the length of each transmissive region P.
FIGS. 4A to 4C are schematic plane views showing crystallization states of an amorphous silicon layer by a sequential lateral solidification method according to the related art.
FIG. 4A shows a crystallization state of an amorphous silicon layer 52 after the first irradiation of the laser beam 34 (FIG. 1) using a mask 62 (FIG. 3). First to sixth polycrystalline silicon regions M1 to M6 corresponding to the first to sixth transmissive regions N1 to N6 of the mask 62 (FIG. 3) are formed in the amorphous silicon layer 52.
In FIG. 4B, the solid line indicates the new position of the mask 62 (FIG. 3) after the first irradiation. The mask 62 or the substrate 44 (FIG. 1) moves along a horizontal direction H (FIG. 3) by a length P (FIG. 3). (The mask 62 moves in the opposite direction to the substrate 44.) After the mask 62 or the substrate 44 (FIG. 1) moves, the laser beam 34 (FIG. 1) irradiates the amorphous silicon layer 52 a second time.
FIG. 4C shows the crystallization state of the amorphous silicon layer 52 after the second irradiation of the laser beam 34 (FIG. 1). The polycrystalline silicon region is enlarged along the horizontal and vertical directions H and V (FIG. 3).
As shown in FIG. 4D, the entire amorphous silicon layer 52 (FIGS. 4A to 4C) is crystallized into a polycrystalline silicon layer 52a through repeating irradiation and movement by the length P (FIG. 3).
In the crystallization process using the mask 62 (FIG. 3) having six transmissive regions N1 to N6 (FIG. 3), the polycrystalline silicon region is enlarged along both of the horizontal and vertical directions H and V (FIG. 3) even though the mask 62 (FIG. 3) moves along the horizontal direction H (FIG. 3). Accordingly, the process time is reduced, and a uniform polycrystalline silicon layer is obtained due to regular irradiation and movement.
However, the mask 62 (FIG. 3) and the crystallization method illustrated in FIGS. 4A to 4D have some disadvantages such as a non-uniformity of crystallization at a borderline between the first and second irradiated regions. As shown in FIG. 4B, because the mask 62 (FIG. 3) moves by the length P (FIG. 3), the first transmissive region N1 does not overlap the first polycrystalline silicon region M1 during the second irradiation by the laser beam. When the laser beam passes through the transmissive region of the mask, the laser beam is distorted by diffraction at a boundary of the transmissive region. Accordingly, the energy of the laser beam is not sufficient for crystallization at the boundary of the transmissive region, and the amorphous silicon layer is not sufficiently crystallized at the boundary of the first polycrystalline silicon region M1. Because the second irradiated region overlaps the first irradiated region along the vertical direction V (FIG. 3), the insufficiently crystallized amorphous silicon layer at the top and bottom boundary is re-crystallized in a subsequent irradiation of the laser beam. However, since the second irradiated region does not overlap the first irradiated region along the horizontal direction H (FIG. 3), the insufficiently crystallized amorphous silicon layer at right and left boundary is not cured and re-crystallized even in a subsequent irradiation of the laser beam. Therefore, the amorphous silicon layer at the borderline k (FIG. 4B) between the first and second irradiated regions has a poor crystallization. When a device such as a thin film transistor is formed at the borderline k, this poor crystallinity of the borderline k causes a reduction of electrical characteristics of the device.